The laser gyro: using light to measure angular velocity
With three decades of research and development efforts to its credit in this field, Thales is one of the only companies in the world to have developed a laser gyro solution performing simultaneous measurements in three axes. Three pairs of lasers (created in a helium-neongas, stimulated by electrical discharge) are maintained in a closed circuit along capillaries, machined in the edges of a regular ctahedronin vitreous ceramic. Each pair comprises two counterpropagating waves, i.e. one wave travelling in the opposite direction to the other. Six mirrors located at the corners of the octahedron cause the light to travel along three square-shaped perimeters. Three of the mirrorsare transmittant (slightly transparent).
When the block is made to rotate in space, under the effect of special relativity, the two lightwaves travelling respectively in the direction of rotation and in the opposite direction are shiftedout of frequency with each other. The prisms arranged in contact with the transmittant mirrors cause a fraction of the two counterpropagating waves to interfere with each other, and photodiodes placed over the prisms observe the interference fringes, which alternate between light and dark, travelling at a speed proportional to the speed of rotation to be measured.
To achieve the degree of sensitivity needed for such measurements (up to one-ten-thousandth of Earth’s rotational speed), the surface state of the mirrors employed must be near-perfect.
Concerning the precision of the direction of travel of the light, the mirrors and prisms are assembled within the block by means of molecular adhesion (pressing parts together until bonds form at atomic level). This feat of engineering and technology is performed on a daily basis at the Thales facility in Châtellerault, France.